Split interpreter into smaller modules (#662)

- Move Heap and Frame/Scope to their own headers.
- Move some functions to more appropriate headers (e.g. CopyValue -> value.h).
- Define a separate Bazel rule for each header/cpp pair.
- Modify PrintValue to not print the frames of a ContinuationValue. This was necessary to break a dependency cycle between PrintValue, PrintFrame, and Action::Print.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/value.h

@@ -240,6 +240,8 @@ struct Value {
 
 void PrintValue(const Value* val, std::ostream& out);
 
+auto CopyVal(const Value* val, int line_num) -> const Value*;
+
 auto TypeEqual(const Value* t1, const Value* t2) -> bool;
 auto ValueEqual(const Value* v1, const Value* v2, int line_num) -> bool;